In a video distribution site that distributes live images, a monitoring system that monitors passengers in a station, etc., images are distributed to a terminal in real time. The quality of distributed images is increasing in accordance with the demand of users who enjoy image distribution services and the volume of image data to be distributed is increasing in order to realize high quality.
A server that distributes images encodes image data and audio data to be distributed and transmits data to be used for multiplexing and synchronizing the audio data and image data to the terminal. A receiving terminal temporarily stores the transmitted encoded data in a buffer and decodes the encoded data when a decoder storage time has elapsed.
However, as the quality of distributed images increases, the decoder storage time at the terminal increases, which increases the delay time. An increase in the decoder storage time increases the wait time until the terminal decodes the distributed encoded data. For this reason, the distributed images are displayed at the terminal late with respect to the images that are actually captured, which impairs synchronization with the live images.
Such an increase in the delay time does not meet the demand of the user and furthermore is problematic depending on the system. For example, in a passenger monitoring system in a station, a delay of an image at a terminal with respect to the actually captured image may cause a delay in finding a passenger who has fallen from a platform. As described above, for realtime image distribution systems, there are demands not only for high quality of distributed images but also for shortening the delay time. The delay time can be calculated with the following Equation (1).delay time=encoding time+network jitter absorption time+decoder storage time+decoding time  (1)
According to Equation (1), the encoding time is a time for encoding data to be distributed at a distribution server. The network jitter absorption time is a time for storing data in packets to be distributed at the distribution server. The decoder storage time is the time from storing received data in a buffer until decoding is started. The decoding time is a time for decoding the encoded data. The decoder storage time has a large share of the delay time and thus shortening the decoder storage time reduces the delay of distributed images.
Patent Document 1: Japanese Patent Application No. 09-298734
Patent Document 2: Japanese Laid-open Patent Publication No. 2002-084339
There is, however, a problem in that it is difficult for the terminal to know the decoder storage time for encoding image data distributed in real time.
When there is little variation in distributed images, because it suffices if a distribution server transmits difference data to a terminal, the volume of data to be distributed is small and, even if the decoder storage time is shortened, there is little effect on the quality of distributed images. However, when the images change suddenly, encoding does not necessarily follow the image change. If the decoder storage time is short, the data for reproducing the images at the terminal is insufficient, which may lead to deterioration of image quality of distributed images. Particularly, because the image quality of distributed images is increasing recently, the volume of image data for reproducing images at the terminal tends to increase.